1. Field of the Invention
The present invention relates to electrophotography and more particularly to apparatus for controlling the rotation of a photosensitive drum in the case of printing a plurality of duplicated copies from the same document.
2. Prior Art
It is known to rotate a photosensitive drum through (n+1) revolutions in the case of printing n copies from the same document. For instance, in U.S. Pat. No. 3,819,261 it is disclosed that in the case of forming n copies using a photosensitive drum having a circumference slightly greater than the length of a document, the drum is rotated through (n+1) revolutions. But in this case, during the first n revolutions, n copies are formed and during the last (n+1).sup.th revolutions, the drum is cleaned in preparation for copying another document. It is also known from a Japanese Laid-open Utility Model Application Publication No. 40,229/75 that when it is required to form n copies from the same document, at the end of an n.sup.th revolution of a photosensitive drum, a delay timer is actuated so as to effect an (n+1).sup.th rotation of the drum.
As disclosed in the known teachings, during the first n revolutions of the drum, n copies are printed and during the last (n+1).sup.th revolution the preparation for a next copying operation is effected. However, such a measure could not be applied to an electrophotographic apparatus having a photosensitive screen drum, because in this case the screen drum should be biased differently between an electrostatic charge latent image forming step and a charge transferring step and thus these steps must be carried out during different revolutions of the screen drum. For example, during a first revolution of the drum, the latent image forming step is effected and during n successive revolutions the latent image transferring step is repeatedly carried out. In this manner the last copy is formed by the last (n+1).sup.th revolution of the screen drum. Therefore, a copy number set dial should not be reset by the first revolution of the drum and the resetting operation should be initiated from the second revolution. By means of the known drum rotation control apparatuses such a control could not be practised.